It is well known that variation in the gasoline volatility can cause major problems with respect to drivability in cold start calibration, when trying to achieve low exhaust emissions. Using a lean start strategy usually causes the problem to increase.
The standard way to solve the problem is to enrich the air/fuel ratio to the extent that most variations in volatility lie within the drivability limits. Such air/fuel ratios will have a rich air factor λ in the range of 0.7-0.9. By definition, an air factor, λ, less than 1 is termed “rich”, while a value greater than 1 is termed “lean”. The air factor is defined as the quantity of intake air divided by the theoretical air requirement, where the ideal stoichiometric air/fuel ratio (14.5 parts air and 1 part fuel) has an air factor of λ=1. The idle speed is conventionally controlled by adjusting the throttle and/or the ignition timing.
Using this rich setting will result in a significant increase in hydrocarbon (HC) and carbon monoxide (CO) in the engine out emission during the critical warm-up phase before the catalyst has reached its operating, or “light-off” temperature. FIG. 1 shows how HC emission increases with a reduction in the air factor, λ.
If the idle speed is set too high in a conventional combustion engine the fuel consumption, and consequently the exhaust emissions, will increase. The driver might also react to the increased noise from the engine. For vehicles with an automatic transmission it causes a noticeable jerking initial movement when the first or reverse gear engages.
If, on the other hand, idle speed is set too low, drivability is affected. Even a small fluctuation in engine stability may cause the engine to misfire, or to stall. The reduced amount of fuel will also increase the time taken for the engine to heat up, which directly affects the time required for the catalytic converter to reach its operating, or “light-off,” temperature.
As a compromise, the engine idle speed is commonly locked to a predetermined value, which a central processing unit (CPU) is mapped to maintain at all times. With the air factor λ set at “rich”, as described above, the CPU uses the throttle and/or the ignition timing to maintain the required idle speed. This rich setting of the engine overcomes problems related to fuel volatility, but precludes a lean start strategy.
U.S. Pat. No. 5,954,025 (TOYOTA) discloses a vehicle with a dual fuel system having a stability detector. This arrangement determines that instability occurs when the engine speed drops below a reference speed, whereby the air/fuel ratio is adjusted. The invention allows variations of the idle speed caused by varying fuel volatility during normal operation, but is not suitable for use with a lean start strategy.
The standard solutions and the above prior art document describe various arrangements for managing engine idle speed, but do not solve the problem of engine emission sensitivity caused by variations in fuel volatility and required torque during a lean cold start, using an air factor λ>1. This problem is solved by the invention as described below.